The present invention relates to a cyclotron with a focussing-defocussing system making it possible to significantly increase the admissible power level, when the target is placed within the accelerating structure.
An isochronous cyclotron is a charged particle (generally proton) accelerator, which comprises an electromagnet incorporating two pole pieces occupying the two axial faces, i.e. perpendicular to the axis, of a relatively flat, cylindrical accelerating cavity. A charged particle source is arranged in the centre of the cavity, in such a way that the magnetic field perpendicular to the path of the particle beam gives the latter a circular path. A conventional acceleration system has two flat electrodes, each of which is constituted by two disk portion-shaped parallel plates. These electrodes are raised to a high alternating voltage making it possible to apply four successive accelerations per revolution to the particles. Under these conditions, the energy of the particles increases and the path of the beam is now a divergent spiral and not a circle bringing the particles up to the periphery of the accelerating cavity. In the case when it is wished to extract the beam from the accelerating cavity (e.g. for bombarding a target so as produce a radioactive isotope), use is made of a curved electrostatic channel raised to a high voltage for the purpose of removing the beam from the attraction of the magnetic field and to tangentially extract it from the cavity. This is one of the most exposed members of the cyclotron and it must be particularly carefully positioned relative to the incident beam, in order to have the best possible extraction efficiency and also for ensuring that it is not bonbarded by an excessive fraction of the incident beam. This requires a very great fineness of the beam, or in other words a good focussing in all directions. The beam is characterized by its horizontal or radial focussing in the plane of the spiral and by its vertical focussing in the direction perpendicular to the pole pieces. It is known that the horizontal focussing can be obtained by increasing the value of the magnetic field as a function of the radius. This field gradient in the cavity also makes it possible to compensate the effect of increasing the particle mass during its acceleration (relativistic effect). However, this gradient is prejudicial to the vertical focussing. This problem has been solved by subjecting the particle, during each revolution, to an alternate succession of strong fields and weak fields. This is obtained by means of pole pieces having a series of thick, spaced sectors, which are circumferentially distributed in order to define in the cavity an alternating succession of wide air gap zones and narrow air gap zones. Under certain conditions, this arrangement ensures the vertical focussing. The power of the extracted beam is defined by the extraction efficiency, which largely depends on the heat dissipation limits of the aforementioned electrostatic extraction channel and more particularly a fine copper strip thereof, which is known as the septum.
However, numerous cyclotron users, particularly for producing certain radioisotopes, wish to obtain the most powerful beam possible. It has then been proposed to place the target within the accelerating cavity, substantially at the same distance from the centre as the electrostatic extraction channel, the latter being provisionally removed from the cavity. In this new cyclotron utilization mode, the pronounced focussing of the beam necessary for the extraction becomes a disadvantage, because the target cannot provide heat dissipation for such a localized impact zone and there is consequently a risk of it being damaged. Therefore the power of the beam must be limited in this internal target utilization mode and it is not possible to use the cyclotron at its nominal power, so that the power gain is below that which could have been expected when used with an internal target.